Friday, April 25, 2008

To ensure it doesn't get too serious around here, here's some Friday afternoon fun:

Personally, I prefer the phrase "snuggle attack" to "corporal cuddling," though the alliteration in the latter is appealing. And, for the record, δ Carina is about 18 inches by at most 5 inches, for an aspect ratio of about 3.6; this is why she has graduated from "furball" to "furslinky."

Tuesday, April 22, 2008

Towards the last week of January, when I was hugely absorbed in trying to figure out a thesis proposal thing, a professor came into my office and started talking about the mass–metallicity relation. I knew about it, of course, but here was a new, simple idea: are the extreme outliers from this locus of galaxies real? In the spirit of a short "one month" project, it's now about three months later, and we've "finally" got a paper on said outliers on astro-ph. (I think this is a great example of: if anyone ever offers you a great idea, take it and run with it.)

So to back up a bit. What's this "mass–metallicity relation"? The short version is that a correlation exists between galaxy mass and metallicity such that the more massive a galaxy is, the more likely it is to be "metal rich." Originally this was the luminosity–metallicity relation, since how bright a galaxy is is much easier to measure than how massive it is, so on the right here I've plotted metallicity versus "absolute magnitude" in small grey points for a large sample of star-forming galaxies (remember: magnitudes are silly, so the right-hand side is brighter than the left-hand side, even though the numbers don't increase in that direction). Our outliers are, well, the larger red and green outliers from this relation; the different colors simply denote slightly different ways of selecting different subsamples. Astronomers being astronomers and not material scientists, when we say "metals" what we really mean is "any element which could not have been made in the Big Bang, i.e., basically anything not Hydrogen or Helium (or maybe Lithium but there's so little of that we'll completely ignore it)." There is of course a nice slew of caveats. The first is that the easiest way to measure the metallicity of galaxies is to limit ourselves to star-forming galaxies; all of those nice new young stars heat up the gas around them, and then as this gas cools it gives off emission lines. We can then look at the spectra of these galaxies and by measuring the how how strong various lines are relative to one another and combining it with some black magic (a.k.a. "spectral synthesis codes") we can measure the ratio of oxygen to hydrogen in a star-forming galaxy's gas. So I (and others!) basically use "metallicity" and "oxygen abundance" interchangeably, and, more precisely, I basically always mean "gas phase" abundance.

One of the interesting interchanges between theory and observation is that sometimes there will be some interesting observation (such as the obervation that galaxy mass and metallicity are correlated). So theorists will come up with a bunch of reasons why this is the case, and a few will even attempt to give explanations fo the scatter about the observed relation. A robust theory will also be able to explain seemingly strange galaxies: if a theory is able to explain why low mass galaxies have such low metallicities, then it should also be able to predict in what ways a low mass galaxy with high metallicity is different from its more mundane cousins. I spend a lot of the paper exploring various explanations (and then eliminating them) for why these galaxies could be so "weird," but I eventually hit upon one explanation which, in retrospect, is blindingly obvious.

The idea is that low mass galaxies have low metal abundances because they have low star formation rates and relatively high gas fractions (i.e., the fraction of their mass that is in gas rather than stars is large). An easy way to think of this is like so: stars turn hydrogen and helium into more massive elements (metals). As stars are formed, they the most massive ones die quickly, throwing their metal-rich selves back into the surrounding gas, thereby raising the metallicity of that gas. But a low star formation rate in a high gas fraction environment will not be making enough metals in order to fully pollute the gas around it, and so the fraction of metals in the gas will be relatively low. (This argument only really works for low-mass galaxies, but since those are the ones we're interested in, I'll ignore that subtlety for now.)

So how do you get high-metallicity low-mass galaxies? Well, presumably the galaxy would need either a very high star formation rate (so the massive stars can actually pollute the gas) or a very low gas fraction (so each supernova has a higher impact on the gas). We find that these outliers don't have unusually high star formation rates, so we conclude that they must have rather low gas fractions. But this is the same gas the stars are forming out of! So the star formation must not have very long left to go. A nice bit of supporting evidence for this scenario is the occasional mention in the literature that so-called "transition" dwarf galaxies tend to have low gas masses and higher-than-expected gas-phase metallicities; these galaxies are known as "transition" objects because they are between regular star-forming dwarfs and quiescent non-starforming dwarf galaxies. The especially neat part is that several of these galaxies have the star formation limited to their centers (like this galaxy to the left: blue in a galaxy is a sign of lots of young, recently formed stars). One way to interpret this is that star formation used to occur on all scales in this galaxy, but the gas has since been extinguished (or blown out of the galaxy) at the larger scales.

Sunday, April 20, 2008

I'm in San Francisco this weekend for my first celebration of Passover. As I've never observed a religious holiday in a purely secular setting (and, no, Christmas doesn't count), this should be fairly fun and educational. Growing up, I always thought Easter and Passover occured around roughly the same time of year, but of course this isn't strictly the case. This year Easter was really early: it falls on the first Sunday after the first full moon after the first day of Spring, which this year all happened to make Easter three days after the Vernal Equinox. And, of course, leave it to an astronomer to explain all of the intricate calendar calculations and relations between Easter, Orthodox Easter, Passover, and Rosh Hoshanah. Basically, the confusing arises when the Vernal Equinox is approximated to be (as opposed to basing these holidays on when the Vernal Equinox actually is).

Wednesday, April 16, 2008

Today is apparently this blog's 2nd birthday. I'm too out of it right now to do anything special, mostly because I finally submitted the paper I've been working on for the last few months to ApJ (i.e., a journal) and the arXiv (it'll be up Thursday night and I'll probably write something real about it this weekend), and I've still go this "thesis proposal" thing to prepare for Friday. So, in the meantime, here is a word count breakdown of the paper. Your job is to guess the subject matter.

Sunday, April 06, 2008

My brother was in town last weekend, and being inspired by my previous two brunch reviews, decided that some "Is it breakfast, or is it lunch?"-y goodness was in order. So last Sunday morning we went over to Spagio's in Grandview, the closest thing I have to a "default" brunch place in Columbus, and this Sunday morning (after visiting for a second weekend in a row!) he has reminded me that I never got around to writing up what I think of brunch at Spagio's.

First off, Spagio's is one of my favorite restaurants in Columbus. It is one of the default places several people in the astronomy department take visitors (colloquium speakers, prospective graduate students, and the like); I unequivocally recommend it for dinner. The restaurant is attached to a decent wine store, so the wine selection is understandably good (and unlike many restaurants, they have a decent list of wines they serve by the glass). The decor is kind of random: there are hammocks hanging from the ceiling (which certainly hundreds of people who have dined there never noticed), random art hanging on the walls (including a weird wooden sculpture of a pig's head and a napkin one of the restaurant's owner's [or the chef's?] friends spilled some wine on and then went to town on turning into Art), and generally just an eclectic collection of decorations which somehow jive together into a neat atmosphere. Pretty much everything I've tried there for dinner I've enjoyed, so it was a natural choice for a first attempt at brunch in Columbus.

So, brunch. Spagio's is a few doors down from Columbus's Stauf's Coffee, and so that is what they serve. Delicious. Unfortunately, this is Ohio, and so no alcohol can be served before 1pm. So much for a bloody Mary for my brother. My normal fare is the croissant French toast with maple cream cheese; oh gods it's so good. Last week I decided to try to jump out of my comfort zone, and I tried the steak and eggs. The steak was fine, but the eggs were kind of flaky and weird and I didn't end up eating them. My brother at least enjoyed his eggs a la maison ("it was food"), but generally I was unimpressed last week. But hey, if someone else is willing to pay for it, I wouldn't turn it down. Just stick to the French toast.

Wednesday, April 02, 2008

This was actually discovered a few months ago, but now there's a press release and even a movie: a peanut-shaped binary star system has been discovered (with the LBT!) in the dwarf galaxy Holmberg IX, a companion to the beautiful spiral galaxy, M81:

And, yes, despite the unfortunate date chosen for the press release, this is a real star, and it is actually exciting. And I take full credit for the term "peanut star." Just please ignore the fact that the movie makes the stars look like they're being externally illuminated instead of, you know, generating their own light.

about me and my blog

I'm a graduate student in astronomy in the cloudy midwest. I have a "cozy" little apartment and an office desk by a big window. If I find something interesting to write about, I usually do; if everything is really boring, I write about that as well (unless I don't).